Electrical resistor and method of making same



May 26, 1953 M. A. THOM ELECTRICAL RESISTOR AND METHOD OF MAKING SAME Filed March 27, 1951 INVE'NTOR.

MELVIN A. Tl-IOM BY A T TORNE'Y Patented May 26, 1953 ELECTRICAL RESISTOR AND METH O D OF MAKING SAME Melvin Arnold Thom, Rochester, N. Y.

Application March 27, 1951, Serial No.217, 785

14 Claims.

The present invention relates to wire-wound electrical resistors and to a process oi manufacturing the same. This application is a continuation-impart of my pending application Serial No. 55,193, filed October 18, 1948, now "Patent No. 2,558,798, granted July 3, 15l.

It is always desirable to keep the weight of any part used in an airplane to a minimum. With the continuously greater and greater heights to which airplanes are required to fly both for commercial and martial purposes, and with the increasingly greater speed at which they fly, resistors of greater and greater capacity are continuouslybeing required. Heretofore an increase in the capacity of a resistor has ordinarily only "been attainable by an increase in size.

Compactness is also afeature desirable in parts employed in an airplane. If they project unnecessarily they get in the way and reduce the amount of available space. Heretofore resistors for usein airplanes have been encased in housings which have required mounting upon a side of the instrument board or chassis of the electrical equipment with which the resistors are to be used.

Resistors employed in naval vessels are always subjected to the corrosive action of salt water, but with the longer and longer tours of duty re quired in'time of war, the problem of protecting the resistors used on a naval vessel against the action of the salt air and spray becomes more more difficult.

One object of the present invention is to provicle a wire-wound electrical resistor in which the windings so well protected that it can be used successfully at high altitudes, as in air- 3 planes, and that it will also be virtually impervious to the attack of sea water and salt air.

Another object of the invention is to provide wire-wound resistor that, while small in size and compact, will be capable of withstanding large electrical loads and high temperatures without fracture or excessive heating.

A. further object of the invention is to afford efficient and durable resistor which is practical economical to manufacture and that is especially for electronic installations in airplanes and applications where conservation of highly essential and where it is necessary for a resistor to have a high currentcarrying capacity so as to conduct such currents r as may be required without the parts becoming excessively heated or causing fracture, damage, disintegration, or lessening of the efliciency of the resistor.

Another object of the invention is to provide 2 aresistor mountedin a'housing of metal or other heat conducting'material in such a Way as 'to-be effectively insulated therefrom electrically but-in such way also asto'permit alarge part of the heat generated by current passing through the resistance wire to be conducted'away and dissipated so that the surfaces of the metal housing and the-surface of the inetalchassis adjacent to the housing cannot become excessively heated, and the resistor is protected'from fracture or th'e damage that might otherwise result from electrical overload or excessively high temperatures.

A further object of the invention is to provide a resistor that may readily be mounted on orremoved from a metal chassis.

Another object of the present invention is to provide an electrical resistor which can be threaded directly into "a hole in thechassis of the electrical equipment on which it is being used. Thus, when the resistor is mounted on the chassis it will project aminimum distance there- "showing further steps in 'th'e'manufacture of this resistor;

Fig. 3 is a sectional View, with parts broken away showing the method of assembling the resistor in its housing, and showing one form of housing constructed according to the present invention;

Fig. 4 is a plan view with parts broken away showing the resistor assembled in the housing and illustrating further steps in the manufacture oi the resistor;

Fig. 5 is a fragmentary plan view, with parts broken away showing the final step in the manuiacture of the resistor;

Fig. 6 is a sectional view on a reduced scale showing one way of "mounting this resistor in a chassis; and

Fig. 7 is a corresponding view showing a modified form of mounting.

In the manufacture of a resistor according to the presentinvention a hollow ceramic cylindrical core l!) is used, the ends of which are closed by metal conducting caps H, only one of which is shown in Fig. 1. Each of these caps has a conducting wire i2 welded or otherwise secured centrally therein to project from its end; and the wires [2 project, therefore, from opposite ends of the resistor body. Each cap is also provided with a plurality of tabs or lugs 14. These tabs or lugs, as disclosed in my U. S. patent application Serial No. 31,236, filed June 5, 1948, now Patent No. 2,524,550, granted October 3, 1950, are equally spaced around each cap. The wire of the resistor is secured at one end to a lug [4 of one cap H, wound on a conventional winding machine around the ceramic body [0, and secured at its opposite end to a lug I4 of the other cap II. As explained in my application above mentioned, the lugs permit of providing a winding of the desired resistance to a very close degree. With this construction of cap and the procedure described, it it possible to construct a wire wound resistor with no substantial variation from the required resistance. This makes it possible to utilize resistors made according to the invention in cases where a tolerance of even a fraction of a percent is all that is permissible.

After the wire 55 is fastened to the caps H as shown in Fig, 1, the attaching lugs I4 are bent over on the cylindrical portions of the caps. Then the entire unit is coated with any suitable insulating and sealing material, such as a silicone varnish or other plastic, so that the coating ll, only part of which is shown in Fig. 1, completely covers and seals the wire winding l5 and also the conducting caps [4 around the core Ill, as shown at the left of Fig. 2, so that the porcelain core or body l0 and the interior of the resistor is effectively sealed against moisture. The wire wound resistor is then baked.

The resistor with its baked coating l! is then placed in a woven sleeve (Fig. 2) made of fiber glass or other suitable insulating material that fits snugly around the coated resistor. Then the sleeve-covered resistor is dipped in an electricallyinsulating silicone varnish, shellac, plastic, resin, or other suitable material and baked. The sleeve 20 is preferably made at least as long as the resistor and is sufllciently porous to begin with so that the varnish, shellac, or other insulating material 2i permeates the sleeve and fills in to some extent the depression between the caps.

A resistor is limited in the load it will carry depending on its size, and in order to accommodate the maximum load required, it is necessary to employ ordinarily much larger resistors than desirable in order to prevent excessive heating, fracture, or other damage. It is essential and important to provide for greater heat dissipation than heretofore possible in order to make a resistor of small size carry maximum loads. To accomplish this, a resistor made according to the present invention is mounted within a rigid housing (Fig. 4) of aluminum or other metal, or suitable heat conducting material, and this housing is constructed for attachment to a metal chassis or support which acts to dissipate readily and effectively the heat generated by any load to which the resistor may be subjected.

In order to insulate the resistor effectively and adequately from the surrounding metal housing and to insure fast and thorough heat dissipation, there is provided a body of electrically insulating material which entirely fills the space surrounding the resistor and between it and the metal housing, this insulating material being applied in such a way as to avoid the possibility of any air pockets or voids remaining in the space surrounding the resistor and between the resistor and the housing.

After coverin the resistor with sleeve 20, dipping the sleeve-covered resistor in an insulating material, and baking it, then, an elongated flexible sleeve 22 (Fig. 3) is fitted over the sleeve 20. This flexible sleeve 22 is also preferably made of woven fiber glass or other suitable electrically-insulating material and is made of a size to fit snugly around the sleeve 20 and over the ends of the resistor. It is also made long enough so that it can be used, as will be described below, as a means for pulling the resistor into its housing 25.

The resistor with the woven fiber glass sleeve 22 in position is then dipped or otherwise impregnated with a silicone or resin varnish, plastic, or other electrically-insulating materia1 to saturate the fiber glass sleeve 22 thoroughly and form a, slight coating thereon. Then the resistor is again baked.

t will be understood that the opening 26 in the housing is of a diameter to accommodate properly the resistor and surrounding insulating sleeves with the plastic material saturating the latter, so that when the resistor unit is inserted in the opening 26 within the metal housing 25 it fits tightly therein. The sleeves 20 and 22 are selected of a size so as to fit closely around the resistor and to fill the space between the cylindrical surface of the resistor and the surrounding surface of the metal housing.

The insulating material being in a plastic state fills in completely any space between the sleeve and the inside of the bore 26 of the housing, as denoted at 21 (Fig, 4), eliminating any possible air pockets or voids.

A filler 34 (Fig. 5) of silicone varnish or other insulating material is then placed over the ends of the housed and sleeve-covered resistor; and the resistor is then heated to say C. to drive oil the solvent. Then ceramic washers 30 are placed over the ends of the housing by threading the ends 32 of the sleeve 22 through central holes in the washers, as shown at the right in Fig. 4; and the washers are forced home, as shown at the left in Fig. 4. The washers cause the sleeve 22 to be tucked in around the ends of the resistor, squeezing the plastic filler 34 around the ends of sleeve and sealing the space between the bore of each washer and the sleeve and at the same time sealing the inside of the turned over portion 35 (Fig, 5) of the sleeve to the end of the resistor. The washers being of material thickness push the varnish up and around the sleeve 22 and complete the seal.

Then the whole unit is baked for say eight hours at say C. and the temperature is then raised to say 250 C. and the bake continued for say one and a half hours. During this time, the plastic coating 20 of the resistor becomes sufficiently plastic to go into and seal any space between the sleeve and the resistor, as denoted at 36 in Fig. 5. Then the ends of the sleeve are cut off, as shown in Fig. 5. The conductor wires l2 are then flattened to form ears 40 and punched to form holes 4| (Figs. 6 and 7) to provide terminals.

The housing 25 is threaded along a portion of its periphery, as denoted at 29. This thread forms not only a means for securing the housing in a chassis, but also serves to dissipate heat.

Fig. 6 shows the completed resistor mounted in position in a chassis or board 45. It is passed sac-cause through a-hole it in the chassis Eandr'is secured: in position by "nuts #41 which :are threaded ton threaded portion 29%of thehousing from opposite endsthereof and which abut atopposite 1 sides against the chassis,

The invention isnot: limited tora housingof the form "described. but Pmay be embodied 'in other forms of housings. Fordnstance, the housing may have a smoothperipheral surfaceaforea'portion of :its length, as dcnotedat 1:58 inal ig. .7 and mayabe3turned down to smaller diameter-along the adjacent portion of its length and threaded, as denoted at 5|. :Such a housing can *bei'fastened in a chassis 45 "by passing :the -=smaller diameter threaded portion i5 I through ithe vhole 46 :in "the housing to abutthe shoulder :48 Fiiormed atithe juncture "of r the 1 larger and? smaller diameter :portions of the resistor againstoneside of the chassis and by threading a nut 4?! i on the threaded vaportionl-fil of-the'resistortoabut this nut d'l a'gainst the. opposite side of=therchassls It is not possible with resistors as heretofore constructed'to mount a resistor on a. metal chassis in an -'electronic installation; 'the resistor 1 must be suspended by rbrackets. With the present structure, however, the resistor :canrbemounted directly in the chassis. Moreover,'-with'the-construction described, a resistor having .an actual length of-approximately one inch and a maximum diameter of. approximately five sixteenths of an'inch will withstand 'electrical iloads that otherwise would requirearesistor of approximately five times that size. Actual tests have shown that in 'areslstor constructed, according to the present invention :and having a "capacity of twentyfive watts, aacurrent load often watts, for instance, produces atemperature at the. outside of the unit of only 40C., whereasa'similar sized resistor of'conventional construction and having no metal housing generates a tempera tureof 250 C. 'when carrying the same load. Other 'tests have shown that with the present structure, a current load of twentywatts produces 'a temperature in :the housing of only 80 C., and a current load of thirty watts produces atemperature in'the housing of"only'1'l9C.

With resistors as heretofore constructed, a rated current load of thirty watts has required a resistor unit three inches lon'g'an'd five-'eig'hths on an inch in diameter, whereasairesistor made according tothe present structure occupying a space of approximately one-fourth i that size can carry a current load of thirty watts an'd will generate a 'highitemperature onithe-outsi de of the housing.0fonly.119 Csas compared with the250 C. temperature generated by the conventional thirty wattunit. The structure of the present invention therefore possesses superiority and practical advantages that have not been possible with any of the :resistoraconstructions heretofore available.

Resistors may be constructed according .to the present invention indifferent sizes for varying capacities. For a five watt rating, :for zinstance, the opening 26 in the metal housingii has a diameterof eleven thirty-seconds of an inch, and the metal wall 1 above the opening a thickness of .O'Zinch.

With a fivewatt unitrsuch as described above, a current of 30.2 watts was required "to raise the internaLtemperature ofwthexresistor toi251C. whereas the temperature :of :a conventional -resistorof thesame size rose .to. 250 'C. with only five watts of current,iandlfifteen'watts rendered the conventional resistor guseless.

While the invention :has been described in connection with dilferent embodiments thereof, it will be understood thatiit .is capable of .further modification, andzthisiapplication is intended to cover; any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from therpresent-disclosure as. come within known 0r:customary:practice;in the art to which the invention 1 pertains and as may be 'appliedto the essential features hereinbefore set rforth and as fall within the scope of the invention or the limits of the "appended :claims.

Having thus describedtmy invention, what I claim is:

'liThe method of producing an electrical resistor which comprises :placing a wire wound body, that has conductors extending centrally outwardly from its ends, in a porous, flexible sleeve madel'of electricalinsulating material and whichislongerthan said body, impregnating said sleeve with an electrical-insulating material, heating :the whole to make the second-named insulatingxmaterial plastic, inserting the whole into a tightly-fitting :bore in a metal housing by first inserting one end of said sleeve into said borerand :then pulling on said one end of said sleevetto pull the whole into said bore, and then baking the assembly.

2. The method of producing'an electrical resistor which comprises placing 'a wire-wound body, that has conductors extending centrally from its'ends, in aporous, flexible sleeve made of electrical-insulating material which is longer than said body, impregnating said sleevewith a liquid electrical+insulating materiaLba-king the whole, heating the whole to render the secondnamed insulating material plastic, inserting the whole, while the second-named insulating material is plastic, into a tightly-fitting bore in ametal housing, which is longer than said body, by first inserting one end of said'sleeve into said bore and then pulling on said one end to pull the'whole into'said bore, then passing each condu'ctor and the corresponding end of the sleeve through a central hole in a washer which is made of insulating material and whose central hole has-a diameter much less than'the diameter of said bore, forcing the two washers into opposite ends of the housing whereby to gather the ends of 'th'e'sleeve over the'ends of the body and about the conductors, and baking the assembly.

3. The method of producing an electrical resister which comprises placing a wire-wound r body, that has cylindrical conductor wires extending centrally from its ends, in a porous, flexible sleeve "made of electrical-insulating material which is longer than said body, impregnating said sleeve with a liquid electrical-insulating material, baking the whole, heating the whole to make the second-named 'lnsu'lating'material plastic, inserting'the whole into a tightly-fitting bore in a metal housing, which is longer than said body, by first inserting one end of said sleeve into said bore and then pulling on said one end to pull the whole into said bore, then passing each'conductor wire and the corresponding'end of the sleeve through a central hole in a'washer which ismade of insulating material and =whose central 'hole has a diameter much less than the diameter of said bore, forcing'the two washers into opposite ends of the housing whereby to gather the ends of the .sleeve over the endsof the :body and :about the conductor wires-cutting offthe endsiof'the sleeve'ito allow the conductor wires to project therebeyond, flattening the extremity of each conductor wire, and punching a hole therein.

4. The method of producing an electrical resistor which comprises coating a wire-wound body, that has conductors extending centrally outwardly from its ends, with an electrical-insulating coating, pulling said coated body endwise into the bore of a metal housing, which is open at both ends and whose length is at least as great as that of the coated body and whose bore will tightly fit the coated body, while the coating is plastic, allowing the conductors to project beyond the ends of the housing, and baking the assembly.

5. The method of producing an electrical resistor which comprises inserting a wire-wound body that has conductors extending centrally outwardl from its ends into a woven glass sleeve, impregnating the sleeve with an electricalinsulating varnish, and, while the varnish is plastic, inserting the whole endwise into the bore of a metal housing whose length is at least as great as that of said whole and whose bore will tightly fit said Whole, and baking the assembly.

6. The method of producing an electrical resistor which comprises winding resistance wire on a core made of an electrical-insulating material and which has metallic caps at its opposite ends from which project outwardly extending conductors, securing opposite ends of the resistance wire to the caps, covering the caps and resistance wire with an electrical-insulating material, inserting the covered whole into a sleeve made of an electrical insulating material and which is longer than the capped core, impregnating this sleeve with a liquid electrical-insulating coating, baking the so-impregnated sleevecovered whole, then heating the so-impregnated sleeve-covered whole to render the coating plastic, inserting it into the tightly-fitting bore of a metal housing which is long enough to enclose the capped core, sealing the ends of the sleeve over the ends of the caps and about the conductors, and baking the housed whole.

7. The method of producing an electrical resistor which comprises placing a wire-wound body, that has conductors extending centrally outwardly from its ends, in a porous fiexible sleeve made of electrical-insulating material and which is longer than said body, impregnating said sleeve with a liquid electrical-insulating material, baking the whole, heating the whole to render the second-named insulating material plastic, inserting the whole, while the secondnamed insulating material is plastic, into the tightly-fitting bore of a metal housing which is longer than said body by first inserting one end of said sleeve into said bore and pulling on said one end of the sleeve to pull the whole into said bore, sealing the ends of the sleeve over the ends of said bod and about the conductors, and baking the housed whole.

8. The method of producing an electrical resistor which comprises winding resistance wire on a core made of an electrical-insulating material and which has metallic caps at its opposite ends from which project conductors, securing opposite ends of the resistance wire to the caps, coating the wire, and caps with a liquid electricalinsulating material, then baking, then placing the baked whole in a sleeve which is made of an electrical-insulating material and which is approximately as long as the capped core, impreghating the sleeve with a liquid electrical-insulating material and baking, placing the sleevecovered whole into a second sleeve which is longer than the capped core, impregnating the second sleeve with a liquid electrical-insulating material and baking, then heating the doublesleeve covered whole to render the last-named insulating material plastic, and, while said lastnamed insulating material is plastic, pulling the double-sleeve covered whole into the tightlyfitting bore of a metal housing which is longer than said capped core, putting a plastic electrical-insulating material into the ends of said housing, threading the conductors and the opposite ends of the second sleeve through two apertured electrical-insulating washers to seal the ends of the second sleeve over the caps, baking the so-sealed whole, cutting off the ends of the second sleeve which protrude beyond the ends of the housing, and forming terminal-attaching portions on the protruding ends of the conductors.

9. A resistor comprising a wire-wound insulated core having conductor wires extending centrally from opposite ends thereof which are electrically connected to the wire-winding of said core, said wire-wound core being covered with an electrical insulating material, a cylindrical metal housing having a bore extending therethrough in which said covered-wire-wound core is mounted with said conductor wires projecting beyond the ends of said housing, said housing being of greater length than said covered core and the bore of said housing being of a diameter to tightly fit said covered core and having its ends sealed to form an air-tight enclosure for said covered core, said housing being externally threaded to be threadably mounted in a support.

10. In combination, a rigid cylindrical metal housing having an axial bore therethrough, a high-voltage electrical resistor having conductor wires extending centrally from the ends thereof, said resistor being mounted within said bore with the conductor wires extending centrally beyond the ends of said housing, said bore being of a size to fit closely about said resistor and the wall of said housing surrounding said bore being of substantial thickness, said housing being externally threaded for mounting in a support, and a body of electrical-insulating material around said resister and entirely filling the space between the resistor and the housing and covering the ends of the resistor and surrounding the conductor wires at the ends thereof adjacent to the resistor.

11. A resistor comprising a wire-wound insulated core having conductor wires extending centrally from opposite ends thereof which are electrically connected to the wire winding of said core, a metal housing having a bore extending therethrough in which said wire-wound core is mounted with the conductor wires projecting beyond the ends of the housing, the bore of said housing being of a diameter to fit tightly said wire-Wound core, electrically-insulating washers which fit around said conductor wires and which close the ends of said housing, and electricalinsulating material around said wire-wound core and entirely filling the space between the resistor and housing and sealing the openings between said washers and the conductor wires.

12. A resistor comprising a wire-wound ceramic core having a metallic cap closing each of its ends, each of which has a conductor wire extending centrally outwardly therefrom, a sleeve made of insulating material covering said wirewound core and said caps and surrounding the conductor wires at the ends thereof which are attached to said caps, a metallic housing having a bore in which the sleeve-covered, capped, wirewound core is mounted, ceramic washers threaded over the conducting wires and the ends of said sleeve and closing the ends of said housing, and an electrical-insulating material impregnating said sleeve and entirely filling the space between said sleeve and said housing and the spaces between the washers and said capped core.

13. A resistor comprising a wire-wound core made of an electrical insulating material having a metallic cap closing each of its ends, each of which has a conductor wire extending centrally outwardly therefrom, a sleeve made of an electrical-insulating material covering said wirecoated core and said caps and surrounding the conductor wires at the ends thereof which are attached to said caps, a cylindrical metallic housing having a bore extending therethrough in which said covered, capped, wire-wound core is mounted with the ends of the conductor wires extending beyond the ends of said housing, washers made of an electrical-insulating material threaded over the ends of said sleeve and said conductor wires and closing the ends of said housing, and an electrical insulating material impregnating said sleeve and entirely filling the space between said Wire-wound core and the housing and between said washers and said caps, said housing being externally threaded for threading into a mounting.

14. A resistor comprising a wire-wound electricallyinsulated core having metallic caps at opposite ends which are electrically connected to the wire winding of said core and which have electrical conductors extending outwardly therefrom, said wire wound core being coated with a baked insulating varnish, a sleeve of electricalinsulating material, which is impregnated with an insulating varnish and baked, covering with said wire wound core, a second sleeve made of an electrical-insulating material covering the first sleeve and the ends of said core and surrounding the conductors at the ends thereof which are attached to said core, a metallic housing having a bore extending therethrough in which the sleeve-covered wire--wound core is mounted with the conductors projecting beyond the ends of the housing, washers made of an electrical-insulating material threaded over the conductors and the ends of said sleeve and closing the ends of said housing, and an electrical insulating material impregnating said second sleeve and entirely filling the space between said second sleeve and the housing and the spaces between the washers and the core.

MELVIN ARNOLD THOM.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,282,398 Ehrlich May 12, 1942 2,387,829 Burnham et al Oct. 30, 1945 2,397,568 Seaman Apr. 2, 1946 2,460,795 Warrick Feb. 1, 1949 2,464,568 Flynn et a1 Mar. 15, 1949 

